v0.9.6: Devel bounds (#60)

* refactor: Use 'args' property instead of '_args' attribute.

As described in Sympy's User Guide, one should never use internal
methods of variables, prefixed with '_' (as an example, it is explicitly
written that one should not use '_args', but use 'args' instead).

Here we apply this rule to the two main modules in the 'topology'
directory: 'basic' and 'domain'.

* refactor: Improve Boundary class

* refactor: Remove obsolete module

* feat: Add coord bounds to Line/Square/Cube domains

Several changes in 'domain' module:

* It is now possible to instantiate objects of type Line, Square and
  Cube (these classes were only used as Domain constructors before);

* Line, Square and Cube are subclasses of the generic NCube class, they
  have different constructor signatures, and use NCube's constructor to
  instantiate an object of their type with minimal code duplication;

* NCube is a subclass of Domain; its constructor returns a Line object
  for dim=1, a Square for dim=2, a Cube for dim=3, and a generic NCube
  for dim>=4;

* Line, Square, Cube, and NCube allow the user to prescribe the
  coordinate bounds, which are (0, 1) by default.

Moreover, unit tests for the new features are provided.

* chore: Do not use latest version of Sympy (1.5)

* refactor: Avoid Codacy errors